Shields served one main purpose at the time of their invention. It was to protect its user from bodily harm by employing it to absorb incoming blows and attacks. Shields never really underwent any radical changes in design though some contained items such as a spike in the outfacing center to act as an additional weapon.
When armor appeared on the battlefields it was nearly impossible to distinguish who was friend or foe. Shields then became the basis for identifying allies. Crests, symbols and icons were painted on the devices to serve as visual representation of which faction its user served.
The above pictured shield is a prime example of one used during the Crusades. Its triangular design was beneficial to the knight as it provided upper body protection as well as mid section defense. The cross that appears on the shield was common during the Crusades as it represented God and the righteous beliefs with which the Crusaders fought their campaign. Note however that in the center of the shield there appears a crest or insignia. This was to identify the knight to his allies in battle.